to make this point extra extra explicit: a life vest is also a "stay very close to the surface" vest. It prevents the worker going down like when you jump into a pool.
The usual reason for this is it keeps your mouth from being far from the air. In this case it also helps because the radioactive stuff is close to the bottom. And exposure depends on distance from the bottom.
the deeper you get, the worse for you. I assume the first second was critical.